Dynamic
1) With Dynamic Routing, routers pass information between each other so that routing tables are regularly maintained.
2) The routers then determine the correct paths packets should take to reach their destinations.
3) Information is passed only between routers.
4) A routing domain is called an Autonomous System, as it is a portion of the Internetwork under common admin authority.
5) Consists of routers that share information over the same protocol. Can be split into routing areas.

Distance Vector and Link-State Routing

Routing Protocols
I) Interior (within an autonomous system – AS – group of routers under the same administrative authority)
a) Distance Vector – understands the direction and distance to any network connection on the internetwork. Knows how
many hops (the metric) to get there. All routers w/in the internetwork listen for messages from other routers, which are sent
every 30 to 90 seconds. They pass their entire routing tables. Uses hop count for measurement. 1) Used in smaller networks
that are have fewer than 100 routers. 2) Easy to configure and use. 3) As routers increase in number, you need to consider
CPU utilization, convergence time, and bandwidth utilization. 4) Convergence is due to routing updates at set intervals. 5) When
a router recognizes a change it updates the routing table and sends the whole table to all of its neighbors.
1) RIP – 15 hop count max
2) IGRP – 255 hop count max, uses reliability factor (255 optimal), and bandwidth
3) RTMP
b) Link State – understands the entire network, and does not use secondhand information. Routers exchange LSP?s (hello
packets). Each router builds a topographical view of the network, then uses SPF (shortest path first) algorithm to determine the
best route. Changes in topology can be sent out immediately, so convergence can be quicker. Uses Bandwidth, congestion for measurement; Dijkstra’s algorithm;
1) Maintains Topology Database. 2) Routers have formal neighbor relationship. 3) Exchanges LSA (Link State Advertisement) or
hello packets with directly connected interfaces. 4) These are exchanged at short intervals (typically 10 sec). 5) Only new info is
exchanged. 6) Scales well, however link?state protocols are more complex. 7) Requires more processing power, memory, and bandwidth.
1) OSPF – decisions based on cost of route (metric limit of 65,535)
2) EIGRP – hybrid protocol (both Distance-Vector and Link State), Cisco proprietary
3) NLSP
4) IS-IS
II) Exterior
1) EGP (Exterior Gateway Protocol)
2) BGP (Border Gateway Protocol)

Problems with Routing Protocols
1) Routing Loops – occur when routing tables are not updated fast enough when one of the networks becomes unreachable. Due to the slow convergence (updates of routing table between all routers), some routers will end up with incorrect routing table and will broadcast that routing table to other routers. This incorrect routing tables will cause packets to travel repeatedly in circles.
2) Counting to infinity – occurs when packets end up in a routing loop; hop count increases with every pass through a router on the networkSolutions to Problems with Routing Protocols1) Define the maximum number of hops – When the number of hops reaches this predefined value, the distance is considered infinite, thus the network is considered unreachable. This does stop routing loops, but only limit the time that packet can travel inside the loop.
2) Split horizon – The packets can not be sent back to the same interface that they originally came from. During the updates, one router does not send updates to the router that it received the information from.
3) Route poisoning – The router sets the cost/distance of routes that are unreachable to infinity. Used with hold-down timers
4) Triggered updates – The router sends updates of the routing table as soon as it detects changes in the network. Does not wait for the prescribed time to expire.5) Hold-Downs – After the router detects unreachable network, the routers waits for a specified time before announcing that a network is unreachable. The router will also wait for a period of time before it updates its routing table after it detects that another router came online (Router keeps an entry for the network possibly down state, allowing time for other routers to re-compute for this topology change). Hold-downs can only partially prevent counting to infinity problem. Prevents routes from changing too rapidly in order to determine if a link has really failed, or is back upnovell-etherEthernet IIarpa (Internet Standard)Snapsnap

Wan Service Providers
1) Customer premises equipment (CPE) – Devices physically located at subscriber?s location; examples: CSU/DSU, modem, wiring on the customer’s location
2) Demarcation (or demarc) – The place where the CPE ends and the local loop portion of the service begins. (Usually in the “phone closet”).
3) Local loop – Cabling from the demarc into the WAN service provider?s central office; wiring from customer’s location to the nearest CO
4) Central Office switch (CO) – Switching facility that provides the nearest point of presence for the provider?s WAN service; location of telephone company’s equipment where the phone line connects to the high speed line (trunk); Regional Telco Office where the local loop terminates (the Telco location nearest you)
5) Toll network – The switches and facilities, (trunks), inside the WAN provider?s “cloud.”

DTE – the router side and receive clocking
DCE – the CSU/DSU side and provide clocking

WAN Devices
Routers – Offer both internetwork and WAN interface controls
ATM Switches – High-speed cell switching between both LANs and WANs
X.25 and Frame-Relay Switches – Connect private data over public circuits using digital signals
Modems – Connect private data over public telephone circuits using analog signals
CSU/DSU (Channel Service Units/Data Service Units) – Customer Premises Equipment (CPE) which is used to terminate a digital circuit at the customer site
Communication Servers – Dial in/out servers that allow dialing in from remote locations and attach to the LAN
Multiplexors – Device that allows more than one signal to be sent out simultaneously over one physical circuit

Switching Terminology
Store-and-Forward ? copies entire frame into buffer, checks for CRC errors before forwarding. Higher latency.
Cut-Through ? reads only the destination address into buffer, and forwards immediately; Low latency; “wire-speed”
Fragment free ? modified form of cut-through; switch will read into the first 64 bytes before forwarding the frame. Collisions will usually occur within the first 64 bytes. (default for 1900 series).

Access ListWildcard MasksAdditional NotesStandard IPSource IP address field in the packet’s IP headerTo put simply, when the IP is broken down to binary, the 1’s allow everything and the 0’s must match exactly.Wildcard mask examples: 0.0.0.0=entire address must match. 0.255.255.255=only the first octet must match, the rest will allow everything. 255.255.255.255=allow everythingExtended IPSource IP or Destination IP, or TCP or UDP Source or Destination Ports, or ProtocolSame as standardThe key word ANY implies any IP value is allowed, the keyword HOST implies the IP exactly has to matchStandard IPXPackets sent by clients and servers, and SAP updates sent by servers and routersConfigured as a hexadecimal number instead of binary-1 means any and all network numbers ( works like ANY)Extended IPXSource Network or Node, or Destination Network or Node, or IPX Protocol, or IPX Socket, or SAPMatch multiple networks with one statement, again in hexadecimalThe most practical use of the protocol type is for NetBIOSSAPSent and received SAP trafficN/AUpdates its own SAP tables. Again uses -1 to mean “ANY”

Accessing Router with Terminal Emulation
Using HyperTerminal on a Windows machine adjust the following settings:
VT100 Emulation
Connection Speed: 9600 Baud
Data Bits: 8
Parity: None
Stop Bits: 1
Flow Control: None
On a Linux machine you may use Seyon or Minicom (at least one should come with your distribution).

Router Startup Sequence
POST
Bootstrap program loaded from ROM
IOS is loaded from either flash (default), TFTP, or ROM
IOS image loaded into low-addressed memory; hardware and software is determined
Config file is load from NVRAM; if no configuration exists in NVRAM, the initial configuration dialog will begin

Miscellaneous Notes
Multiple Loop Problems ? complex topology can cause multiple loops to occur. Layer 2 has no mechanism to stop the loop. This is the main reason for Spanning ? Tree Protocol.

Virtual LAN?s (VLAN’s) ? sets different ports on a switch to be part of different sub-networks. Some benefits: simplify moves, adds, changes; reduce administrative costs; have better control of broadcasts; tighten security; and distribute load. Relocate the server into a secured location.